Multifrequency signaling is an inband interoffice address signaling method in which ten decimal digits and five auxiliary signals are each represented by a pair of signals selected from a group of at least six distinct tone frequency signals. This signaling method is now commonplace in communication systems. Multifrequency signaling is employed in subscriber signaling, signaling between central offices, intraoffice communications, remote control of other systems, control of remote test equipment, imputting data to computer systems, and the like. Consequently, it is increasingly important that detection of valid multifrequency signals be achieved accurately and inexpensively.
Multifrequency receivers utilize a plurality of narrow bandpass filters and a plurality of corresponding threshold comparators to detect valid multifrequency signals in a received signal. Threshold comparisons are performed with respect to either a fixed or variable threshold level. In the variable threshold level type of multifrequency receiver, the threshold level is dynamically generated in response to the received multifrequency signal. See, for example, U.S. Pat. No. 4,227,055 issued to R. L Hanson on Oct. 7, 1980. Each threshold level represents an instantaneous amplitude of the received signal. When the received signal ceases to be present, the threshold level returns to a predetermined quiescent level.
Each bandpass filter in the multifrequency receiver is characterized by a narrow passband centered about a particular tone frequency. To avoid erroneous detection of out-of-band signals, the bandpass filters are designed to have an extremely steep rolloff characteristic at the cutoff frequency.
One problem with prior multifrequency receivers which employ these bandpass filters in combination with a variable threshold level generation circuit is that transient disturbances in the nature of superimposed, damped oscillatory signals appear at the filter outputs following an abrupt input stimulus such as an on-off transition of the received signal or a burst of shot noise or the like while the variable threshold level is at or returning to the predetermined quiescent level. The magnitude of the disturbances exceeds the variable threshold level by an amount sufficient to cause the threshold comparators to indicate the presence of signal levels exceeding the threshold level. Hence, the multifrequency receiver erroneously detects the transient disturbances from the bandpass filters as a valid received multifrequency signal.